Spark plug with auxiliary gap



April 25, 1950 K. cHwARTzwALDER ETAL 2,505,150

SPARK PLUG WITH AUXILIARY GP Filed March 17, 1945 Patented Apr. 25, 1950 SPARK PLUG WITH AUXILIARY GAP Karl Schwartzwalder, Holly, and Robert W. Smith, Flint, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application March 17, 1945, Serial No. 583,364

(Cl. 12S-169) 8 Claims. 1

This invention has to do with spark plugs having an auxiliary spark gap incorporated in the center electrode.

It has long been known that the use of lan auxiliary spark gap in series with gap of the spark plug will enable it to function effectively long after it would otherwise have been shortcircuited by fouling-that is by the accumulation of carbon and other conducting combustion chamber products on the firing end of the plug forming a conducting path between the electrodes. This action is increased by placing the auxiliary gap as close as possible to the spark gap of the plug because the distributed capacity of the portion of the center electrode between the ltwo gaps is thereby reduced to a minimum producing a steep wave front at the spark gap so that there is insufficient time for leakage of current through the path produced by fouling to prevent the spark from jumping the gap.

An object of the present invention is to provide an auxiliary gap of durable construction that may be readily incorporated in the center electrode of a spark plug and will operate at substantially constant voltage through the life of the plug. Another object of the invention ls the provision of a simple and practical center electrode construction embodying an auxiliary spark gap.

In the drawings:

Figure l is a vertical sectional view through one form of spark plug embodying my invention.

Figure 2 is a vertical sectional View through a modiiied form of spark plug embodying my invention.

In Figure l, I indicates the lower part of the metal shell of the spark plug having secured to its lower end the side electrodes indicated at I2. Within the shell is the insulator I4, preferably of aluminum oxide composition. The insulator is held in its seat in the shell by any suitable means such as by annular C-shaped spring I6, metal washer I8 and the upper part 20 of the shell which is threaded in the lower part I0 and may also be welded or brazed thereto. Suitable gaskets are interposed between the shoulders of the insulator I4 and the shell I0 and C-shaped spring I6 to distribute the pressure evenly on the insulator. The upper portion of the shell is provided with the usual sleeve 22 of insulating materia] held in place by any suitable means. Insulator I4 is provided with a central bore 24 having an enlarged upper end and a reduced lower end. Fitted within the reduced lower end is the lower electrode section 26 which may be made of metal, such as nickel, which is resistant to heat as well as to spark erosion. This portion of the electrode has an enlarged upper end which seats on the shoulder formed at the junction of the enlarged and reduced portions of the insulator bore 24, and is preferably of hollow construction with the center lled with metal such as copper, indicated at 28, having good heat conductivity.

Above the lower electrode section is the conducting glass seal 30; above that a resistance element 32; and above the resistance element a second conducting glass seal 34 on which is seated the disc 42 of the lower electrode 44 of the auxiliary gap. An additional glass seal 31 engages and grips the lower threaded or serrated end of the upper electrode element 38.

The auxiliary spark gap assembly 36 is of simple construction consisting of a sleeve 40 of insulating material, preferably steatite, although aluminum oxide or other ceramic compositions may be employed. Steatite is preferable because it may be readily cut to the desired size. The sleeve should have a rough surface to reduce the possibility of short-circuiting by deposition of the metal sputtered from the electrodes. Each end of the cylinder is closed by a disc 42 having secured centrally thereto a cylindrical electrode section. The discs 42 preferably consist of heat resistant conducting material, such as commercial nickel, and the electrodes 44 are preferably made of tungsten because of its excellent resistance to heat, spark erosion and oxidation. The tungsten contacts may be secured to the nickel discs by copper brazing or the like. Other materials suitable for use as electrodes although inferior to tungsten are platinum, platinum tungsten alloy, nickel and other alloys commonly used for spark plug electrodes. Copper will be found satisfactory where lower fabricating and operating temperatures are involved.

The auxiliary spark gap should be of such length that the voltage across it is substantially the same as that across the gap of the plug. As the pressure at the auxiliary gap is atmospheric and much lower than in the combustion chamber the auxiliary gap distance should be much greater than the spark gap.

The conducting glass seals 30 and 34 and the resistance element 32 may be of the construction described and claimed in the prior application of McDougal, Schwartzwalder and Rulka, Serial No. 488,114, filed May 24, 1943, now U. S. Patent No. 2,459,282, issued January 18, 1949. Thus the seals 30 and 34 may be made of a fused mixture of 55 parts powdered copper, 45 parts Pyrex Percent S102 34 B203 28 PbO 29 A1203 7 NaaO 2 It will be noted that this seal contains no organic binder corresponding to the Dritex used in the conducting glass seals 30 and 34. It has been found that when a binder is present in the glass seal compositions during the fusing of the glass in the insulator bore a carbon lm is deposited on the insulating sleeve 40 of the spark gap, which lowers the insulation resistance of the auxiliary spark gap assembly and may cause the gap to be inoperative. However, when the binder is omitted the powdered material is diflic-ult to handle in the small amounts required. Consequently instead of omitting a binder entirely from the composition of' the conducting glass seals 30 and 34, the binder is employed to form the powdered material into pellets which are subsequently sintered, preferably in a hydrogen atmosphere, at from 1500 to 1'7007 F. for from fteen minutes to one hour. The resulting pellets are substantially free from the binder and may be readily handled in manufacture.

In assembling the electrode in the insulator the lower electrode element 28 is inserted in the bore followed by the powdered or pelleted materials constituting the lower conducting glass seal 30; then by the pulverized or pelleted material forming the resistor 32 and then by the pulverized or pelleted material forming the upper conducting glass seal 34. It has been found best to use either all powdered material or all pelleted material for the parts 30--32-34 as the combination of powdered and pelleted material does not produce the desired final structure. Thereafter the spark gap assembly, the glass seal 31 and the upper electrode section 38 are inserted and the assembly is then heated to a sufficient temperature to soften the glass of the seals and resistance element whereupon the upper electrode section 38 is pressed down to cause it to seat on the upper disc 42 and make a tight assembly. On solidiflcation the conducting glass seals 30 and 34 form an air-tight union with the walls of the bore in the insulator and grip the other parts with which they make contact.

An important feature of the invention is the character of the glass seal 31. It has been found that when a gas-tight seal is used at this location the voltage across the auxiliary spark gap increases as the plug is used because of disappearance of the air from the gap. The preferred glass composition of the seal 31 produces a crazed or finely cracked glass which, while securely united to the insulator and the cooper'- ating parts, nevertheless permits seepage of air into the auxiliary gap. With this construction the voltage drop across the auxiliary gap has remained substantially constant throughout the life of the plug.

In the modification shown in Figure 2 the construction of the shell and the manner of mounting the insulator in the shell is substantially the same as that previously described. The electrode, however, consists of a platinum tip 50, the upper end of which is slightly enlarged and rests on a shoulder provided in the bottom of the bore 52 in the insulator. The section 54 of the electrode consists of silver cast in place. Above the silver is the glass seal 56 which may be of the copper glass composition described with the Dritex eliminated by baking the glass pellets. If desired, a resistance may be incorporated in the glass seal as disclosed in Figure l. 58 indicates the upper section of the electrode, the lower end of which rests on the upper disc of the auxiliary gap assembly indicated at S0. The electrode section 58 may be corrugated or grooved as in the preceding form to'better interlock with the glass of the seal 51 which is of the same -composition as the seal 3l of Figure 1.

Various modifications will occur to those skilled in the art. Thus, for example, compressed powdered talc could be used in place of the glass seals, suitable means being provided to hold it under compression. The upper seal instead of consisting of crazed glass permitting the entrance of air could be provided with a vent for the same purpose.

Various other modifications will occur to those skilled in the art.

We claim:

l. In a spark plug an insulator having a bore therein, a lower electrode assembly sealed in the lower end of the bore, an auxiliary spark gap assembly in the bore in series with the electrode assembly, an upper electrode section in the bore in series with the said gap, and means pervious to air chemically bonded to the insulator, securing the upper electrode section in place and permitting of seepage of air into said gap.

2. In a spark plug an insulator having a bore therein, a lower electrode assembly sealed in the lower end of the bore, said assembly including a resistance to reduce spark erosion, an auxiliary spark gap assembly in the bore in serieswith the electrode assembly, an upper electrode section in the bore in series with the said gap, and means pervious to air engaging the interior of the bore and the upper electrode section, securing the latter in place and permitting of seepage of air into said gap.

3. In a spark plug the combination of an insulator having a bore therein, an electrode sealed in the lower end of the bore, a resistance in series with the lower electrode, an auxiliary spark gap assembly in the bore in series with the resistance, an upper electrode in the bore in series with the spark gap assembly, and means pervious t0 air bonded to the insulator securing the upper electrode in the boie and permitting of seepage of air into said gap.

4. In a spark'plug an insulator having a bore therein, said bore having secured therein in electrically conducting series relation a lower electrode, a conducting seal, a. resistance and a second conducting seal, said seals and resistance spanning the bore and bonded to the insulator, an auxiliary spark gap assembly, an upper e1ectrode in conducting engagement with the spark gap assembly, and means permitting seepage of ai; securing the upper electrode in the insulator 5. In a spark plug an insulator having a bore therein, said bore having secured therein in electrically conducting series relation a lower electrode, a conducting seal, a resistance and a second conducting seal, said seals and resistance spanning the bore and bonded to the insulator, an auxiliary spark gap assembly, an upper electrode, and means pervious to air surrounding the upper electrode, securing it in place and permitting of seepage of air into said gap.

6. In the combination as defined in claim 2, said last-named means comprising a crazed ceramc seal.

'7. In the combination as defined in claim 5, said last-named means comprising a crazed glass seal.

8. In a spark plug an insulator having a bore therein, a lower electrode assembly sealed in the lower end of the bore, an auxiliary spark gap assembly in the bore in series with the electrode assembly, an upper electrode section in the bore in series with the said gap, means pervious to au: bonded to the insulator, securing the upper electrode section in place, said last named means comprising a crazed ceramic seal.

KARL SCHWARTZWALDER. ROBERT W. SMITH.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS 

